Tree Risk News

What do the Boffins have to say?
Whilst putting VALID together with the Risk Professor, and Bath University, we learned many lessons about risk modelling. One lesson learned was that I’m an Arborist. I mainly know about trees. There’s much I didn’t know about risk modelling. Okay, I did know risk matrices are fundamentally flawed and can’t sensibly rank risks. And you can’t apply mathematical rules to ordinal rankings. Though I was yet to find out how much I didn’t know, I was at least smart enough to realise some people knew much more than me.

How uncertain do we need to be?
Another important lesson learned was there’s too much uncertainty in tree risk to claim single figure value accuracy. It’s not credible to measure tree risk to such accuracy as 1/4, 1/300, 1/20 000, or 1/5 000 000. Nor can you realistically measure the difference between a 1/10 000 and a 1/50 000 risk. And you really shouldn’t be compounding the error by adjusting these very accurate risks by double or single figure multipliers like 0.25 or 2, 3, or 4.

With tree risk, we’re looking to measure something with high uncertainty, and our risk ratings should reflect that. With VALID there are only four tree risk ratings. None of them is a single figure value.

Here’s our Drive-by Assessment specification that we've been working on the Tasmanian Government's Department (DSG) of State Growth.  This is now embedded into the Government Agency Tree Risk-Benefit Management Strategy template at Appendix A2.

DSG are working on their Traffic Management Plan and I'll encourage them to share that as well. This is really important because the risk of assessing trees on a high-use road is much higher than the risk of branches or trees falling on a high-use road.

It can be adapted to meet your needs. By way of explanation for how this is set up. It’s part of the Government Tree Risk-Benefit Management Strategy. So, you’ll need to be familiar with that to understand the difference between Passive Assessment & Active Assessment. And the three levels of Active Assessment – Basic, Detailed, and Advanced.

Zones of high confluence = High use + large trees

In VALID that’s 1400 or more vehicles a day. The speed limit doesn’t matter. For people, we’re looking at around 1200 per day. This translates to someone passing every minute or so between 7am – 7pm, Mon – Fri.  Large trees, for purposes of zoning rather than assessment are trees with a DBH of about 50cm/20in or more.

Drive-bys can be carried out by a Basic Validator or a Validator.  This is released under a creative commons licence, and you're welcome to replace these trained assessor titles with whoever would be carrying out a Drive-by Assessment and what their credentials are.

Putting the risk from trees falling into perspective.

A neat and easy to understand relative risk came up in Tim Harford’s marvellous Cautionary Tales - The Spreadsheet of Life and Death. Driving for about 400km/250mi has a risk of death of about one in a million (1 micromort). This everyday risk is so low it's one we readily accept.

We know the overall risk of death or serious injury from branches or trees falling each year is less than one in a million (less than 1 micromort).

That means your risk of dying in an car accident on a 400km/250mi round trip to visit friends for the weekend, is higher than being killed by a tree over the whole of a year.

*Micromort = a unit of risk that's a one-in-a-million chance of death. Micromorts are useful unit of measurement that allow us to compare the risk from day-to-day activities.

A tree which has a risk that's so high it's Not Acceptable or Not Tolerable will usually be large, have an Obvious Tree Risk Feature, AND be in a high-use zone.

Tree Risk Management is how a Duty Holder goes about managing the risk from branches or trees falling to an Acceptable or Tolerable level.

Trees give us many benefits that we need. But, they're natural structures that sometimes fall over or shed branches; usually because of severe weather.

Tree managers and owners have a Duty or Standard of Care to manage the risk from their trees. The Duty also says they should be reasonable, proportionate, and reasonably practicable when managing the risk.

That means there's a balance they need to strike between the many benefits trees provide, the risk, and the costs of managing the risk. By taking a balanced approach, they don't waste resources by reducing risk - and losing benefits - when the risk is already Acceptable or Tolerable.

The most effective way for a Duty Holder to discharge their Duty of Care is to adopt a Tree Risk-Benefit Management Strategy, that includes a Policy and a Plan.

Compared to other everyday risks we readily accept, the overall risk to us from branches or trees falling is extremely low.

Our annual risk of being killed or seriously injured is less than one in a million. That's so low, we're at greater risk driving on about a 400km/250mi round trip to visit friends for a weekend than from branches or trees falling over an entire year. 

Driving for about 400km/250mi = 1 micromort, which is a one in a million chance of dying.

Given the number of trees we live with, and how many millions of us pass them daily, being killed or injured by a tree is a rare event. A rare event that usually happens during severe weather.

Tree risk is about the risk from branches or trees falling.  It has three variables to it.  Two likelihood variables and one consequences variable.

1) The likelihood of a branch or tree falling

2) The likelihood of someone or property being there when the branch or tree falls

3) The consequences of the tree part hitting someone or property

What is the risk from branches or trees falling?
Compared to other everyday risks we readily accept, the overall risk to us from branches or trees falling is extremely low. Our annual risk of being killed or seriously injured is less than one in a million. That's so low, we're at greater risk driving on about a 400km/250mi round trip to visit friends for a weekend than from branches or trees falling over an entire year. Given the number of trees we live with, and how many of us pass them daily, being killed or injured by a tree is a rare event; one that usually happens during severe weather.

Duty holders who don't have a strategy explaining how they manage the risk from tree failure are vulnerable to legal claims or enforcement action. Even though we know the overall risk of death or serious injury from tree failure is extremely low, a number of recent Coroner's Inquests from around the world have highlighted why having a strategy in place is so important.

Death on the highway, from the Arboricultural Association's Arb Magazine (Summer 2020), takes a closer look at the importance of Tree Risk-Benefit Management Strategies to the duty holder.

A case of rough Justice?

Here's a LinkedIn article about the landmark Cavanagh v Witley Parish Council Judgment. It explores the gulf between reasonable, proportionate, and reasonably practicable tree risk management and assessment, and expert evidence in some UK court Judgments.

We now have a Cavanagh v Witley Parish Council Update

Every summer, when we get long hot dry periods, concern is often raised about the risk from Summer Branch Drop (SBD).  Fear not.   We’ve got the risk management of SBD covered for you in our free Summer Branch Drop (SBD) Guide.

Is this SBD?

In brief, the overall risk from SBD is mind-boggling low. What that means is there’s no need to fret about putting up signs, or fencing, or pruning, unless you have a tree that’s a repeat offender.

Have a look at our Risk Management page for lots more free and handy common sense tree risk management advice and help.